The present invention relates to a drive apparatus for a color cathode ray tube, and in particular to a drive apparatus having an automatic white balance adjustment circuit and a color picture tube which is effective to prevent a reference signal for white balance adjustment as a raster from being displayed thereon.
In a display such as character graphic display, TV receiver or monitor TV receiver which uses a multi-color picture tube a white balance adjustment is performed by an automatic white balance adjustment circuit as follows. A cut-off adjustment is performed by inputting a white signal representative of a relatively dark white to the automatic white balance adjustment circuit as a reference signal for white balance adjustment for adjusting a direct current level of the inputted signal so that the ratio of cathode currents corresponding to current red, green and blue become predetermined ratios. A drive adjustment is performed by inputting a white signal representative of relatively light white to the white balance adjustment circuit to adjust the gains of respective amplifiers so that the ratios of cathode currents corresponding to current red, green and blue colors become predetermined ratios. The white balance adjustment is thus performed by the automatic white balance adjustment circuit so that no color is displayed at various areas having various brightness on the screen of the color picture tube when white and black video image is reproduced. The present invention is concerned with a drive apparatus for a color cathode ray tube having a white balance adjustment capability which is obtained by performing at least the above mentioned cut-off adjustment.
There has heretofore been a problem that a white balance of color picture tube which has been adjusted on shipping from a factory is readily changed after a long term use. This is caused by change with time due to decrease in electron emission from cathodes and by drift of circuits. An automatic white balance adjustment circuit in which change in white balance is recovered is described in, for example, Japanese Unexamined Patent Publication No. Sho. 60-18087 specification entitled "Color television receiving system".
FIG. 1 is a block diagram showing such a prior art automatic white balance adjustment circuit.
In FIG. 1, R (red), G (green) and B (blue) primary color signals which are inputted to input terminals 1R, 1G and 1B, respectively pass through signal synthesizing circuits 8R, 8G and 8B, respectively and are amplified by drive adjustment variable gain amplifier circuits 10R, 10G and 10B, and then are level-shifted level compensation circuits 11R, 11G, and 11B for cut-off adjustment. The signals are amplified to an amplitude which can drive a picture tube 6 by video output circuits 12R, 12G and 12B and are supplied to the picture tube 6 through cathode currents detection circuits 9R, 9G and 9B.
An automatic white balance control adjustment performed at this time will be described hereafter with reference to signal waveform at various parts shown in FIG. 2.
Signals represented at (b) and (c) in FIG. 2 which are extracted from a composite video signal to be displayed as a video image are applied to vertical and horizontal blanking pulse input terminals 3V and 3H of a signal generation circuit 2 for automatically adjusting white balance, respectively. Two signals 4B for cut-off adjustment and 4W for drive adjustment represented at (d) in FIG. 2 which are generated from these signals (b) and (c) are inputted as reference signals for white adjustment to the signal synthesizing circuits 8R, 8G and 8B via a signal line 4.
If one (composite video signal) of the inputted three R, G and B primary color signals is assumed as (a) in FIG. 2, the output from corresponding signal synthesizing circuit (8R if 8R) will have a signal wave form as (e) in FIG. 2. If the signal (e) in FIG. 2 corresponds to the primary color B signal, a detected voltage which is proportional to a cathode current from a cathode 7B flowing into the emitter of a transistor 28 in a cathode current detection circuit 9B is inputted to a sampling circuit 13 via a cathode current detection signal line 30B. Since circuits 9R and 9G are identical with the circuit 9B in structure, the circuits 9R and 9G are simply designated as blocks for simplicity of illustration. This is same as the circuits 12R, 12G and 12B.
Gate pulses which are in synchronization with the reference signals 4B and 4W are supplied to the sampling circuits 13R, 13G and 13B via a gate signal line 5. A negative feedback action by a cut-off adjustment comparator (or operational amplifier) 16B and a drive adjustment comparator (or operational amplifier) 17B determines an optimum adjustment level in a level compensation circuit 11B for a cut-off adjustment level and an optimum adjustment level in a variable gain amplifier circuit 10B. Control voltages in the sampling circuit 13B at this time which correspond to the optimum adjustment level of the level compensation circuit 11B for cut-off adjustment and the optimum adjustment gain of the gain variable amplifier circuit 10B are held in a hold capacitor 15B for cut-off adjustment and a hold capacitor 14B for drive adjustment, respectively, and are then supplied as control voltages to comparators 17B and 16B, respectively until next sampling time.
Reference voltage sources 17 and 18 are used for controlling the cathode currents on cut-off and drive adjustment to prescribed values, respectively. The above mentioned operation is same in the R and G primary color signal circuits.
The reference voltage source 17 is commonly used for comparators 16R, 16G and 16B corresponding to three primary colors. The reference voltage source 18 is commonly used for comparators 17R, 17G and 17B. The detected voltage values representative of the cathode currents which are inputted to the inverting terminals (-) of respective comparators via sampling circuits 13R, 13G and 13B are preset at predetermined ratios necessary for three primary color signals so that cathode currents for respective colors can be controlled at ratios necessary to keep the white balance. In other words, white balance can be stabilized by controlling cathode currents of respective primary colors for the picture tube on insertion of the reference signals.